64 Mr. W. J. Sollas on two new and 
same time it has become shorter ; and in fig. 25 the diminution 
in length has gone a step further, while the shaft has become 
straight and cylindrical, so that, but for the absence of a ter- 
minal mucrone, it would almost exactly resemble the typical 
spicule of C. mucronata. In the last-mentioned species we 
shave the spicule of fig. 4 showing a very considerable shorten- 
ing in the long direction ; in fig. 5 the spicule tapers from its 
wide neck, instead of enlarging from a constricted neck 
towards its rounded extremity. Fig. 2 appears to be inter- 
mediate between figs. 1 and 5. 
These mucronate spicules are quite distinct from any form 
of spicule yet figured or described ; and it is therefore exceed- 
ingly interesting to find varieties of them in which the mucrone 
is changing its character, and, by enlargement, tending towards 
the fusiform outline of C. ens’fera. The instances in which 
this change is well marked were not discovered till after the 
plates were drawn; and soa sin- | 
gle example is represented in Fig. 1. 
the woodcut (fig. 1). Here,then, 
while C. ensifera shows spicules (ron earning 
tending towards C. mucronata, 
C. mucronata ee the other hand Variety of mucronate spicule of 
presents us with a variety of © mucronata. x 435. 
its staple spicule which almost 
passes into the staple spicule of C. ensifera. 
But the difference between the other spicules of these two 
species, viz. the slender acuates (figs. 6, 12) and the flesh- 
spicules (figs. 9, 15), is so slight that no one would think of 
founding species on them alone. ‘The main difference be- 
tween C. mucronata and C. ensifera exists only in the form 
and size of the staple spicules (fig. 2, 11) ; and this distinction, 
as we have already indicated, is half or more than half bridged 
over by varietal modifications. It hence appears to me that 
in these two forms of Cliona we may actually witness, so to 
say, the transformation of species ; for of the claims of each of 
the species we have described to distinction no one can for a 
moment doubt, while, at the same time, the forms by which 
one might pass into the other are also sufficiently obvious. 
The flesh-spicules of both species are very interesting, as 
they appear to exist in all stages of growth. Their first appear- 
ance, so far as I can make out, is in the form of a simple 
straight rod about 0:0004 inch in length; this soon becomes 
sinuous and spined at each end with three or four conical 
spines; additional spines then appear along its sides, while 
by unequal lateral growth and by the unequal development 
of lateral spines the multicurved forms (figs. 15, 18) are 
